Methods that assist the patient proportionally to his or her own respiratory effort and relieve the patient of the increased work of breathing in order to thus prevent exhaustion of the respiratory muscles and the so-called respiratory failure have been developed in recent years for respirating spontaneously breathing patients. Compared to conventional forms of respiration, the form of respiration called proportional assist ventilation offers a relief for the respiratory muscles, it guarantees a physiological breathing pattern and increases the patient's comfort, e.g., due to improved sleep.
Two fundamentally different concepts are known for the proportional assist ventilation methods: the so-called “Proportional Assist Ventilation” (PAV) and methods with “Naturally Adapted Ventilatory Assist” (NAVA).
In methods with “Proportional Assist Ventilation” (c.f., e.g., Younes, M.: Proportional Assist Ventilation, in: Tobin M. J., ed.: Principles and practice of mechanical ventilation, New York, McGraw-Hill, 1994, pp. 349-369), a pressure assist is generated, which contains a percentage proportional to the currently present volume flow (flow) as well as a percentage proportional to the volume. The degree of assist is preset by the setting values Flow Assist (FA) and Volume Assist (VA). Due to the positive feedback of the volume flow and the volume, this form of respiration embodies a kind of servo control, which makes it possible separately to compensate percentages of the resistive and elastic resistances of the breathing system and thus to quantitatively relieve the patient of the work of breathing. However, a sufficiently accurate estimated value must be available for this for the actual resistance® and elastance (E), because instabilities (so-called run-aways) and possible damage to the lungs due to barotrauma may otherwise develop.
Furthermore, efforts have been made for quite some time now to determine R and E during spontaneous breathing reliably and in a minimally invasive manner (cf., e.g., WO 97/22377 A1). The special difficulty is due to the fact that the patient's spontaneous breathing activities may cause great errors in determining the breathing technical parameter. A usual procedure is the introduction of interference maneuvers into the breathing pattern (e.g., by a short-term occlusion) at points in time at which a passive phase of breathing is suspected, and the subsequent analysis of the disturbed respiratory signals. However, it is not guaranteed that the patient is in an undisturbed phase of the breathing cycle at the time of the maneuver, and the validity of the measurement is therefore not guaranteed; it also cannot be demonstrated later. This is due to the circumstance that the activity of the respiratory muscles cannot be separated from the mechanical respiration pattern based on close correlations either on the basis of signal theory or statistically.
In methods with “Naturally Adapted Ventilatory Assist” (NAVA), as described, e.g., in: Sinderby et al.: Is one fixed level of assist sufficient to mechanically ventilate spontaneously breathing patients?, Yearbook of intensive care and emergency medicine, 2007, Springer 348-367; Sinderby et al.: Neural control of mechanically ventilation in respiratory failure, Nature Medicine, 1999 (5), 12: 1433-1436, the electrical activity of the diaphragm (EAdi) is recorded by means of a modified gastric probe equipped with electrodes in order to regulate the pressure assist of the ventilator in proportion to this electrical activity. Interference signals (e.g., ECG) are filtered out in advance. The advantages of NAVA are improved interaction between the patient and the ventilator due to synchronized ventilation and the physiological breathing pattern associated therewith. It was demonstrated in a more recent study (Sinderby et al.: Inspiratory muscle unloading by neurally adapted ventilatory assist during maximal inspiratory efforts in healthy subjects, Chest, 2007, 131: 711-717) that a relief of the work of breathing is achieved by an adapted setting of the so-called NAVA level (amplification factor that defines the pressure level relative to EAdi) and overexpansion of the lungs is avoided, because the EAdi signal decreases at high NAVA level. As a result, the risk of run-away decreases. Unlike in the case of the usual pressure assist, there are no fundamental problems with the triggering of a respiration stroke (“triggering”) in patients with dynamic hyperinflation (e.g., in patients with chronic obstructive pulmonary disease (COPD)), because a possible intrinsic peak end-expiratory pressure (PEEP) represents no obstacle thereto. Termination of the inspiration (“cycling off”) is likewise unproblematic.
One drawback of the NAVA method is that an invasive gastric probe is necessary. Patients who would especially benefit from the use of NAVA methods (e.g., patients with COPD subjected to long-term noninvasive respiration, i.e., respiration with a mask), will dislike accepting such a permanent solution. Furthermore, it is not possible to make a distinction between a situation in which the EAdi signal is compromised for technical reasons inherent in the device (e.g., due to interference signals or faulty coupling between the signal source and the electrodes) and a situation in which the respiratory drive decreases. The requirement that the patient be relieved of a certain amount of work of breathing can therefore be achieved only qualitatively. If the coupling between the signal source (EAdi signal of the muscles) and the electrodes changes, this affects the work of breathing to be performed by the patient. The respiratory pressure is usually controlled in direct proportion to the EAdi signal processed. If the EAdi signal increases (e.g., due to increased respiratory drive), the increased respiration by the ventilator leads, on a rather long-term basis, to a reduction of the respiratory drive and correspondingly of the EAdi signal. This negative feedback becomes stabilized at a certain EAdi signal level. If the coupling between the signal source and the electrodes changes—e.g., deteriorates—in this equilibrium, the tidal volume is reduced, the drive and the respiratory effort gradually increase, but the measured EAdi signal decreases, which means that the percentage of the work of breathing performed by the patient increases in an undesired manner.
A special proportional assist NAVA method using a signal for the electrical activity of the diaphragm is known from U.S. Pat. No. 7,021,310 B1; the peculiarity of this method is that the electrical activity of the diaphragm, which is needed for a certain tidal volume (the so-called neuroventilatory efficiency), is said to be maintained at a constant value by means of a “closed-loop” control. In case the patient's properties in terms of the mechanics of breathing worsen, the respiratory effort (and hence the activity of the respiratory muscles) increase to maintain the tidal volume per unit of time. The “closed-loop” controller would counteract this by increasing the assist, so that the EAdi signal level remains unchanged and overloading/exhaustion of the patient is avoided. This method is disadvantageous in the case that frequently occurs in reality, in which the signal properties (especially the amplitude) of the measured EAdi signal change, e.g., decrease due to a change in the coupling between the electrodes and the signal source (caused, e.g., by repositioning of the patient). The controller of the “closed-loop” system would consequently erroneously reduce the assist, because the neuroventilatory efficiency has seemingly increased.